Boiling of liquids occurs in numerous industries including chemical processing, electric power generation, and food preparation. As heat is applied to the surface of a vessel containing a liquid, the temperature of the liquid rises to the saturation temperature or boiling temperature of the liquid. When the liquid as a whole is slightly below the saturation temperature, a thin layer of liquid near the heated surface can rise above the saturation temperature and initiate boiling. Bubbles of vapor will form in the liquid, but they collapse when they travel to the cooler portion of the liquid. The boiling process when the bulk liquid is below the saturation temperature is termed "subcooled boiling." (For purposes of this discussion the term simmering will be defined as the condition when a liquid is at or on the verge of subcooled boiling.) When the temperature of substantially all of the liquid reaches the saturation point, vapor bubbles will rise to the liquid surface. This process is termed "nucleate boiling," or, more commonly, "full boiling," or "rolling boiling."
In steam generation plants, the boiling conditions are generally determined by measuring the pressure and temperature of the fluid and comparing the measurements to standard operating conditions of the plant. In batch chemical reactors boiling conditions are not as easily determined. Temperature measurement alone is insufficient as a boiling condition detector because a fluid at its saturation temperature can be boiling or condensing. A pressure measurement may not be available if product gasses are vented. An acoustic boiling detector that can determine the condition of boiling independently of temperature and pressure measurements would be useful in these reactors.
Concerning the particular application of cooking appliances, different apparatus and methods have been proposed for detecting the cooking conditions in such appliances. These apparatus typically include a means for controlling the rate of heating of cooking appliance surface units, such as natural gas burners or electrical elements, in response to particular detected cooking conditions. U. S. Pat. No. 3,828,164 to Fischer et al., for instance, proposes detecting the steam of boiling water and generating and transmitting a control signal by radio in response to detection of the steam-generating cooking condition. The invention has the limitation that a sensor must be placed above the cooking vessel. This entails a specialized lid for the cooking vessel, with associated expense and cleaning difficulties. Furthermore, the invention can detect only whether the liquid being cooked is producing steam; it cannot detect when subcooled boiling is in process.
U. S. Pat. No. 4,481,409 to Smith proposes the use of weight detection and rate of weight loss computation methods for controlling the rate of heating of an electrical surface unit in a cooking appliance. This invention provides a means only for inferring the cooking or boiling conditions from the rate of weight loss, but the sensing method cannot operate properly if the cooking vessel is covered by a lid, preventing the evaporating steam from escaping. Furthermore, the weight loss at a simmer condition is minimal, and this invention is ineffective in detecting this condition.
To overcome equipment and processing limitations associated with the known prior art, we have discovered that acoustic energy emission detection techniques and appropriate signal processing may be advantageously utilized to detect thermodynamic conditions such as no boiling, subcooled boiling, and full boiling. The detector we have invented can be used to inform an operator of the current boiling conditions, or to activate a control system to maintain or change boiling conditions.